When heating a metal strip in a heat treatment furnace, heating is generally performed indirectly using radiant tubes. In such indirect heating, thermal inertia is high, such that effective heat input to the metal strip becomes more difficult the smaller the difference between the temperature of the metal strip and the furnace temperature, resulting in productivity constraints. Moreover, in such indirect heating, it is difficult to achieve rapid heating in the vicinity of a transformation point at which a heat absorbing reaction occurs, and it is also difficult to achieve high temperature annealing due to constraints in the heat resistance of the radiant tubes. The degree of freedom when selecting heat treatment conditions for metal strips is therefore constrained.
By contrast, in induction heating, the metal strip is heated using high frequency current, and the heating speed and heating temperature can be freely controlled. Induction heating consequently offers a high degree of freedom in heat treatment operations and in the development of metal strip products, and is a heating method that has been garnering attention in recent years.
There are two main methods of induction heating. One method is a longitudinal magnetic flux (LF) heating method in which a high frequency current is passed through an induction coil surrounding the periphery of a metal strip, causing magnetic flux to penetrate a length direction (direction of progress) cross-section of the metal strip (a cross-section taken orthogonally to the length direction of the metal strip). This generates an induction current perpendicular to the magnetic flux and running in a loop within the length direction (direction of progress) cross-section of the metal strip, thereby heating the metal strip.
The other method is a transverse magnetic flux (TF) heating method in which inductors (strong magnets) wound with primary coils are placed on both sides of the metal strip, and current is passed through the primary coils to generate magnetic flux that penetrates a strip face of the metal strip via the inductors, generating an induction current in the strip face of the metal strip, and thereby heating the metal strip.
In LF induction heating, in which induction current runs in a loop within the length direction (direction of progress) cross-section of the metal strip, due to the relationship between the permeation depth δ of the current and the current frequency f (δ (mm)=5.03×105√(ρ/μr·f), wherein ρ (Ωm): specific resistance, μr: specific magnetic permeability, f: frequency (Hz)), if the permeation depths of induction currents generated at front and back faces of the metal strip are greater than the thickness of a steel sheet, the generated induction currents interfere with each other, with the result that induction current is not generated within the length direction (direction of progress) cross-section of the metal strip.
For example, in the case of non-magnetic metal strips, steel sheets that lose their magnetism on exceeding their Curie temperature, or the like, the current permeation depth δ becomes deep, and so induction current is not generated if the strip thickness of the metal strip is thin. Moreover, even in the case of magnetic metal strips, for example, if the strip thickness is too thin in comparison to the permeation depth, induction current is not generated within the length direction (direction of progress) cross-section of the metal strip when using the LF method.
By contrast, in TF induction heating, since the magnetic flux penetrates the sheet faces of the metal strip, the metal strip can be heated irrespective of the strip thickness, and whether or not the metal strip is magnetic or non-magnetic. However, there is an issue with TF induction heating in that overheating is liable to occur at ends of the metal strip (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-151245).
In normal TF induction heating, there is also an issue that it is difficult to adapt to changes in the strip width of the metal strip, since it is not easy to change the shape of the inductors facing the strip faces of the metal strip.
Accordingly, for example, Japanese Patent Application Publication (JP-B) No. S63-027836 describes an electromagnetic induction heater provided with magnetic pole segments that are disposed side-by-side in a width direction of a thin sheet so as to face the sheet faces of the thin sheet, and are capable of moving independently in a strip thickness direction of the thin sheet, and a movable shielding plate of a non-magnetic metal, that is capable of deployment in the sheet width direction of the thin sheet and that adjusts the magnetic field of the magnetic pole segments.
This electromagnetic induction heater is capable of adjusting the magnetic flux according to changes in the sheet width of the thin sheet. However, it is difficult to adjust the magnetic flux in the sheet width direction rapidly when there is a large change in the sheet width of the thin sheet.
Japanese National Phase Publication No. H11-500262 describes a transverse magnetic flux induction heating system provided with plural independent magnetic rods, and a variable width magnetic circuit capable of adapting to the strip width of a metal strip. However, in this induction heating system, induction coils are integrated together with the magnetic rods, and so it is difficult to adjust the magnetic flux in the strip width direction if the strip width of the metal strip exceeds the induction coils. Moreover, it is difficult to adjust the magnetic flux in the strip width direction if the strip width of the metal strip is less than the sum of the width of the magnetic rods.
Moreover, JP-A No. 2002-8838 describes an induction heating device including plural magnetic rods. In this induction heating device, the plural magnetic rods are configured so as to be capable of moving in the strip width direction of a metal strip. This thereby enables changes in the strip width dimension of the metal strip to be accommodated by adjusting the spacing of the plural magnetic rods. However, in this induction heating device, the number of the magnetic rods disposed facing the metal strip is fixed even when metal strips have different width dimensions. Metal strips with different width dimensions are accommodated solely by adjusting the spacing of the magnetic rods. The following issue is therefore conceivable. Namely, when heating a metal strip having a broad strip width, the number of the magnetic rods facing the metal strip is fixed, and when there is a large change in the strip width of the metal strip, the spacing of the magnetic rods becomes larger. In other words, a gap between the magnetic rods in the strip width direction of the metal strip becomes larger. Since no magnetic rods are disposed in this gap area, there is a tendency for the heating temperature to decrease at a portion of the metal strip corresponding to the gap. As a result, there is a possibility of the heating temperature becoming uneven in the strip width direction of the metal strip.